Known electronic cameras include an image sensor which comprises a plurality of light-sensitive elements—so-called pixels—which are arranged in rows and columns and which convert light incident through an objective of the camera into electrical signals. Each of the pixels is addressed to read out an image, with each pixel corresponding to a respective picture element of the image. A signal which is proportional to a charge of the pixel collected by an exposure is conducted to an output of the image sensor.
Sensors are in particular known which have a separate row selection line for each row and a separate column line for each column. Such an image sensor is shown in FIG. 1. The reading out of the image sensor 1 shown in FIG. 1 takes place row-wise, i.e. row for row. For this purpose the pixels 2 of the respective row is connected to the column lines 5 by means of the respective row selection line 6. A separate column amplifier 4 is associated with each of the column lines 5 to amplify the signals of the pixels 2 of the selected row applied to the column lines 5. In FIG. 1, the column amplifiers 4 are not shown individually, but rather in the form of a coherent block which symbolizes the column amplifiers 4 arranged in a row. The amplified signals are then conducted via a multiplexer device to the output or, if a plurality of outputs are provided, as is preferred to achieve a high picture rate, to the outputs of the image sensor. Furthermore, a row addressing logic 3 can be recognized to address the row selection line 6 associated with the respective row to be read out.
The speed at which an image can be read out of the image sensor in accordance with FIG. 1 is determined inter alia by that time which the signal of a pixel needs to move from the output of the pixel along the respective column line to the respective column amplifier.
With image sensors having a large image format of for example, 20 mm image height, the column lines of the image sensor are approximately 20 mm long and have a capacity of, for example, 10 pF. The pixels are typically only a few μm large. There is therefore only a little space available for each pixel. Since the light-sensitive element of a pixel should be as large as possible to achieve a high light sensitivity, the other elements of a pixel have to be made all the smaller, i.e. small transistors have to be used. Such transistors need, for example, 2 μs to drive the capacity of a column line. With correlated double sampling, two reading processes take place per picture element to be read out since a reference signal is additionally read out beside the actual signal for the suppression of the thermal noise. The reading of the signal values of a row then takes 5 μs, for example. The reading of an image with, for example, 200 lines thus requires a total of 10 ms so that the picture rate is limited to 100 frames per second.
To achieve a higher picture rate, it is known to divide the column lines in the middle as is shown in the image sensor 1 in accordance with FIG. 2. The pixels 2 of a respective column are therefore divided into a first pixel group, a lower pixel group in FIG. 2, and into a second pixel group, an upper pixel group in FIG. 2, with the pixels 2 of the respective first pixel group in each column being coupled to a common first column amplifier 41 via a common first column line 51 and the pixels 2 of the respective second pixel group being coupled to a common second column amplifier 42 via a common second column line 52. Since the column lines 51, 52 of the image sensor 1 in accordance with FIG. 2 are only half as long as the column lines 5 of the image sensor 1 in accordance with FIG. 1, they also only have approximately half the capacity of the column lines 5 of the image sensor 1 in accordance with FIG. 1. In addition, two respective rows, namely a row from the lower image field half and a row from the upper image field half, can be read out simultaneously. The possible picture rate can thereby increase to, for example, 300 frames per second.
The signals of the pixels 2 of the image sensor 1 in accordance with FIG. 2 are therefore read out over two rows of column amplifiers 41, 42 which are arranged at the lower edge and at the upper edge of the image field of FIG. 2. The image to be read out and the pixel field are thereby split into two blocks 10, 11 such that the pixels 2 of the lower block 10 are associated with the lower row of column amplifiers 41 and the pixels 2 of the upper block 11 are associated with the upper row of column amplifiers 42. The separation of the two blocks 10, 11 is illustrated by a dividing line 7.
As a rule, the individual column amplifiers cannot be manufactured completely identically with one another. It is rather the case that the properties of the column amplifiers vary slightly from one another. For example, an offset voltage, an amplification or a drift can be slightly different for different column amplifiers. The picture elements associated with the respectively deviating column amplifier then appear somewhat brighter or darker in comparison with the other picture elements. These slight deviations are as a rule not perceived by the eye, however, if such slight deviations occur distributed statistically or randomly over the whole image.
If, however, the mean value of the deviation over the upper column amplifier row deviates systematically from the mean value of the deviation over the lower column amplifier row, for example due to a deviation of one or more process parameters in the manufacture of the column amplifiers, the border formed between the two blocks can be perceived as interference which is horizontal in FIG. 2 and runs through the image center, in particular because the human eye amplifies linear structures.